1. Field of the Invention
The present invention relates to a method for monitoring the transmission quality in a cellular radio communication system which supports plural signal coding and decoding methods, hereinafter termed codec modes, for radio transmission in each cell.
2. Description of the Related Art
In radio communication systems, items of information such as, for example, speech, picture information or other use data are transmitted by electromagnetic waves over a radio interface between a transmitting and a receiving radio station, such as for example a base station or a mobile station in the case of a mobile radio system. The transmitting and the receiving station each have an encoder/decoder, termed codec for short, which serves to convert, for example, a binary coded digital data stream to be transmitted into a coding which is suitable for radio transmission, or to convert back a data stream received by radio into its original coding.
The information transmission takes place here in so-called frames, i.e., all data are combined within a frame duration of, e.g., 20 ms, and are coded/decoded in common.
Numerous signal coding and decoding methods, or codec modes, are known, which are respectively optimized for different transmission conditions on the radio path between transmitter and receiver.
Because of the mobility of the subscribers, the transmission quality on the radio path in mobile communication systems can vary markedly over short periods. The use of different codec modes makes possible satisfactory speech communication even under these variable conditions. Different codec modes use different methods for the data compression and data reduction which convert a speech signal into a digitized data stream with respectively different data rates. After the radio transmission, the digitized data stream is converted back again in to a speech signal at the receiver. The higher the data rate of the digitized stream, obviously the better the accuracy with which the original signal can be reproduced at the receiver. It would therefore be basically desirable to be able to transmit the digitized speech signal at as high a rate as possible.
The data rate which can be transmitted on a conventional radio communication network is of course insufficient for speech transmission with high fidelity in real time. In known GSM mobile radio systems, this data rate is, e.g., 22.8 kb/s per channel. The problem arises there that the radio transmission of data, particularly in mobile radio networks, is error-prone, since the transmission conditions between transmitter and receiver are very variable due to the mobility of the subscriber, and it frequently happens that a data radio signal radiated from a transmitter reaches the receiver over several paths of different length, so that the signal received by the receiver is composed of several components which have an a priori unknown time and phase offset from one another.
In order to be able to transmit an intelligible speech signal under these conditions, it is necessary to transmit in the available channel not only the digitized speech data stream, but furthermore additional information which is generated by the transmitter from the speech data stream and which makes it possible for the receiver to recognize whether data was or was not erroneously received, and if necessary to reconstruct this data. It goes without saying that both the transmission of the speech data stream and also the data transmitted for its reconstruction, termed protective data, is error-prone. Hence the amount of protective data needed in order to be able to satisfactorily reconstruct a given amount of use data is greater, the greater the error rate of the transmission.
Since the total transmission capacity of a channel is limited and is to be always completely used as far as possible, the ratio of speech or general use data to protective data can only be varied by setting the portion of the total bandwidth of the channel available to the use data to be low under poor transmission conditions and high under good transmission conditions. In order to be able to produce the speech data stream with a variable data rate corresponding to the available bandwidth, respectively different data compression and reduction methods or codec modes are used.
The GSM AMR codec is known from GSM Specifications 06.71 and 06.90 for transmitter and receiver of the GSM system, and supports 8 codec modes (CM) for full rate transmission and 6 codec modes for half rate transmission. Each of these modes converts a speech signal into a digitized data stream and is optimized for a given range of transmission quality.
When the transmission quality of the channel changes, the codecs of the transmitter and receiver are changed over to the appropriate codec mode for the present quality. This changeover takes place in that the receiver, when it considers the codec mode used at present not to be optimum, transmits to the transmitter an instruction to change over to another codec mode, designated in the instruction. The AMR codec uses two kinds of such instructions. One includes two bits, which respectively permit specifying one out of four possible codec modes, at a predetermined place of a speech frame. These four modes are termed a codec mode set. The composition of the codec mode set is arbitrary per se and is currently established by the network operators based on experiential values.
The number of codec modes supported by the AMR codec is however greater than four. Therefore not all of the codec modes can be selected in the manner described above. When a change is to be made to a codec mode which does not belong to the present codec mode set, a so-called escape frame (ER), which makes it possible to select another codec mode set, has to be transmitted with this. This escape frame is transmitted instead of a speech frame, and thus leads to a worsening of the speech quality. When the codec mode sets are unfavorably composed, it can happen that frequent changes between two sets frequently have to be made.
It is not immediately possible to determine an optimum composition of the codec mode set which makes it possible to minimize the number of required escape frames. Since the local boundary conditions which determine the transmission quality are respectively different from one cell of a mobile radio network to another, it cannot be concluded, from the fact that a given codec mode set is very suitable for a first cell and a high percentage of codec mode changes which occur in this cell take place within this set, that the same set is also suitable for a neighboring cell. An added difficulty is that the transmission conditions within a cell are not alone determined by the geographical circumstances of the cell. Thus the transmission quality of a terminal device which is situated at a given location of the cell can depend strongly on which time slot is allocated to it, because individual time slots at this location are possibly strongly affected by interference due to a neighboring cell, while others are not. The same holds for the different transmission frequencies used within a cell. Therefore, even within a cell, a codec mode set which is suitable for a first time slot may be found to be unsuitable for a second time slot.
In order to ensure each time a correct decoding at the receiver of the data transmitted by radio, a further two bits are transmitted with each speech frame, and give the codec mode used by the transmitter, in that they specify one of four codec modes of the codec mode set which is valid at any given time.
There are thus two types of codec mode. Information or CM information which is transmitted between terminal devices and the base station in the cells of a radio communication system is of a first type, which informs one partner of a connection of the codec mode used by the other partner, and a second, which informs him which codec mode the first partner wishes to receive from him.
A further problem of conventional cellular radio transmission systems is the lack of an easily operated method which makes it possible to gain exact conclusions concerning the distribution of the transmission quality within geographical partial regions, such as for instance individual cells of the system, and to obtain data concerning the transmission quality respectively related to these partial regions, which data can be consulted as a basis for planning the further extension of the radio communication network or of its cells.